Nanomaterials inferior to 100 nm are constituent blocks of a new and recently emerging subject area, designated nanotechnology. In fact, an enormous interest all over the world is now observed towards the “very small”. From scientific areas to politics itself, nanotechnology is also frequently present in main world leaders' speeches. It is presented as holding the potential of providing a technological discontinuity with as relevant positive implications in citizen life as past historical events, such as the discovery of steam machines, the train, the automobile, the computer and biotechnology. Such enthusiasm is a result of particles or structures, such as films, pores or nanometric surfaces within a given material, presenting a group of electrical, optical, magnetic and mechanical properties, which are different from those of the same material having superior dimensions. In particular, we refer to properties such as high plasticity, super hardness, lower melting point, transparency and high surface area per mass unit with a consequent improvement in catalytic activity, low thermal conductivity, an increase on the magnetic effect, high semiconductor luminescence, color alterations, and inclusively quantum mechanics laws compliance. Some scientists inclusively propose a theory on the probable state of matter.
In recent years, and as a result of the knowledge on these new properties, a thorough investigation has been observed towards the development of new nanomaterial production having production yield at an industrial scale (ton/day) which supports a safe large-scale incorporation of these materials in multiple applications. The known methods, which are thoroughly referred to in the literature, are divided into three major categories:
I—Liquid-Phase Methods
This category comprises a group of methods already established or in industrialization imminence, namely: a) sol-gel, b) co-precipitation and c) hydrothermal and electrochemical synthesis. These have as common principle a starting material in solution or gel form, wherein precursors are either dissolved to a molecular scale or dispersed in the desired stoichiometric proportion. In a following stage, these precursors are decomposed in controlled manner, a precipitate being formed, usually as an hydroxide, which requires several subsequent treatment stages, such as calcination, for its conversion into an oxide with the desired crystalline structure, being followed by a final breakdown by grinding process.
The major drawback/limitation in these liquid-phase methods, apart from low production rates, are associated with high liquid/solvent amounts, which must be later removed or eliminated so as to maintain the nanoparticles in low aggregation. In this removal process, the solvent must be carefully and meticulously separated from the nanoparticles, thus implying compulsory recycling and treatment systems for liquid effluents, which consume plenty of energy and require complex equipments of time consuming operation. Furthermore, by increasing the surface area per mass unit associated to nanomaterial, its solubility (including that of some oxides) considerably increases, thus causing toxicity problems in resulting effluent liquids.
III—Gaseous-Phase Methods
These methods comprise processes for both the production of individual nanoparticles and for direct application in surface coating, namely a) combustion synthesis, b) spray pyrolysis, c) evaporation/oxidation of metals, plasma, CVD, PVD, laser deposition, etc. Generally, these methods are based on the vaporization of the precursors introduced in a variety of solvents. After turning into the gaseous state, the chemical intended reaction among precursors occurs, followed by nanoparticle condensation, with the consequent heat release, which implies the inevitable and undesirable coalescence and coagulation stages of the formed nanoparticles, thus causing aggregate formation and therefore showing a major drawback of this method.
Once the first stable molecules of condensed nanoparticles come from the gaseous state, this methodology is usually designated by “bottom-to-top” approach, that is, it starts from the individual molecule to a first stable structure.
The most common nanomaterials obtained in gaseous phase and for long commercially available are silica and titanium dioxide (pigment), both resulting from the hydrolysis of respective chlorides. The decomposition of the later also brings forth complicated environmental issues associated to the production of chlorine and hydrochloric acid as reaction by-products. On the other hand, despite their large surface areas, the high agglomeration degree in these materials obstructs their use in applications (non-catalytic) requiring non-agglomerated particles.
II—Solid-Phase Methods
In this category, nanoparticles are usually prepared from a first slow reaction in the solid state among different precursors, such as carbonates, oxides etc. It is also designated “mechanosynthesis”, wherein the reaction activation energy is supplied by a mill, being followed by an intensive grinding process until particles inferior to 200 nm are obtained. The main limitations in this low-cost method, besides the difficulties observed in obtaining dimensions inferior to 0.2 microns, are related to the presence of impurities, with a non-homogeneous particle-size distribution and essentially at a deficient homogeneity degree, especially when it comes to synthesizing composites and ternary structures or superior structures, originated by incomplete diffusion reactions among reagents. From a conceptual perspective, and unlike previous methods, this is a “top-to-bottom” approach wherein the starting point is a micrometric structure, whose dimension will be consecutively reduced by mechanical energy application.
The use of the emulsion detonation concept as nanomaterial synthesis method is disclosed in a set of recent documents:
EP1577265, “Production of fine powder of aluminium oxide” discloses an industrial process for the production of micrometrical alumina from a cyclic detonation method of mixed granulated aluminum with an oxidizer, the later possibly being an emulsion (w/o). This has distinct steps, including the previous preparation of a class-1 substance (explosive) and wet collection: it essentially implies feeding the granulate material into the reactor; being followed by the explosion; the products thus obtained passing to the wet chamber; subsequent cooling and final collection stage of the material. The process thus described differs from that described in the present invention in that the process is carried out in gaseous phase and does not include insoluble precursors, either stable or instable when in contact with water.
WO2009040770 “Nanocrystalline spherical ceramic oxides, process for the synthesis and uses thereof” discloses a synthesis process for spherical micrometrical particles, with nanocrystalline structure, from the detonation of an emulsion (w/o) at a detonation temperature superior to the melting point of the oxides thus formed, allowing these to assume a spherical form.
This is a synthesis method exclusively carried out in gaseous phase and mainly using soluble precursors or metals. This process only allows obtaining micrometric dimension oxides. This process differs from the technical characteristics of the solution described by the present invention in that it is carried out in gaseous phase and does not include insoluble precursors, either stable or instable when in contact with water.
WO2009144665 “Nanometric-sized ceramic materials, process for their synthesis and uses thereof” discloses a method for nanomaterial synthesis, such as binary, ternary and higher oxides, nitrates and carbonates, from an emulsion (w/o) detonation at low temperatures (inferior to the ceramic nanomaterial melting point), with dissolution of soluble metallic precursors in oxidizing phase (internal), or from the addition of soluble propellants to the external phase or addition of metals or metal alloys, after emulsion formation. This is a synthesis method exclusively carried out in gaseous phase and mainly using soluble precursors or metals similarly to the methods described in the preceding documents, therefore differing from the method described by the present invention which is based on a solid-phase synthesis resorting to insoluble precursors, either stable or instable when in contact with water.
Xiao Hong Wang et al. (Nano-MnFe2O4 powder synthesis by detonation of emulsion explosive. Applied Physics A: Materials Science & Processing. Vol. 90, no. 3, March 2008) discloses nanoparticle synthesis of a ferrite (MnFe2O4) from the detonation of an emulsion (w/o) ignited by a military explosive (RDX), wherein the precursors (iron nitrates and manganese, respectively) were previously dissolved in the internal phase. Just as with the previous methods, this is a synthesis method exclusively carried out in gaseous phase and mainly using soluble precursors in oxidizing phase of the emulsion, consequently differing from the method herein disclosed.
Xinghua Xie et al. (Detonation synthesis of zinc oxide nanometer powders. Materials Letters, Vol. 60, issues 25-26, November 2006. Pp 3149-3152) discloses a process for obtaining ZnO and Li2O nanoparticles from the detonation of an emulsion, in which lithium and zinc nitrates were dissolved in the internal phase. This emulsion is later ignited by a no. 8 detonator, placed inside a military explosive (RDX). Just as with the previous methods, this is a synthesis method exclusively carried out in gaseous phase and mainly using soluble precursors in oxidizing phase of the emulsion, which also resorts to the use of explosive material (class 1), RDX and detonators. This process differs from the technical characteristics of the solution described by the present invention in that it is carried out in gaseous phase and does not include insoluble precursors, either stable or instable when in contact with water.
The process of the present invention is generally within the scope of “solid-phase methods” and aims at overcoming the limitations associated with this category of methods, namely: difficulty in obtaining dimensions inferior to 0.2 microns, impurities present in the emulsion, time-consuming reaction times and low homogeneity degree in composites or ternary structures, due to incomplete diffusion reaction among reagents, upholding the use of common and cheap insoluble precursors, such as carbonates and metallic hydroxides.